||The purposes of this study are to understand the variations of the particulate and dissolved Pb-210 and Po-210 in profiles in the South China Sea and to estimate the residence time of these nuclides through their extent of radioactive disequilibrium. The profiles of Pb-210 and Po-210 in particulate and dissolved phases were measured at four stations in the northern South China Sea (station B, C, D and F) and three in the Bashi Channel (stations W2, W3 and W5) from four Ocean Researcher I cruises (ORI-575, ORI-580, ORI-588 and ORI-606). At Station C two profiles were taken, one before and the other after a typhoon to see its effect, if any, during the ORI-588 cruise. As the associated Ra-226 profiles were not measured their relations to Pb-210 profiles are not available for discussion in this thesis.|
The mean suspended particulate matter (SPM) concentration for each station is about 0.2 mg/kg with small deviations. Activities of Pb-210 and Po-210 in the northern South China Sea were all higher than those in the sea off NE Taiwan easured during the KEEP program. The dissolved Pb-210 profiles generally display a maximum around 15 to 25 dpm/100kg in the surface layer due to atmospheric input, and a minimum of about 5 to 15 dpm/100kg from 300 to 1000 m. The activities below 1000 m vary only slightly with a deep maximum as high as 20 dpm/100kg around 2000 to 3000 m, as seen at Station C. The particulate Pb-210 is around 3 to 5 dpm/100kg with small variation for the entire water column. The dissolved Po-210 profiles are somewhat similar to the dissolved Pb-210 profiles, but lower by about 5 to 10 dpm/100kg in activity. The mean residence time is about 0.76 year for total Po-210 in the entire water column, and 0.35 to 2.13 years for total Pb-210 in the mixed layer. At each station, Po-210 is deficient relative to Pb-210 in either the particulate or dissolved phase. The mean total
Po-210/Pb-210 activity ratio is about 0.58, indicating that Po-210 is more effectively scavenged. It is imperative to understand the fate and pathway of the missing Po-210. Judging from its biophilic nature, Po-210 may have been absorbed or consumed by biomass and enriched in
various tropic levels in the food chain, in addition to being scavenged and removed by sinking particles.